Martensitic stainless steel for brake disk and method for producing said steel

ABSTRACT

A martensitic stainless steel used for a brake disk of a two-wheeled vehicle includes: in % by mass, C of 0.025% to 0.080%, Si of 0.05% to 0.8%, Mn of 0.5% to 1.5%, P of 0.035% or less, S of 0.015% or less, Cr of 11.0% to 13.5%, Ni of 0.01% to 0.50%, Cu of 0.01% to 0.08%, Mo of 0.01% to 0.30%, V of 0.01% to 0.10%, Al of 0.05% or less, and N of 0.015% to 0.060%; a DFE value defined by a formula (1) ranging from 5 to 30; and a δ ferrite fraction observed in a cross section structure ranging from 5% to 30% by an area ratio. Ti, B, Nb, Sn and Bi may be added. 
         DFE =12(Cr+Si)−430C−460N−20Ni−7Mn−89  (1)

TECHNICAL FIELD

The present invention relates to a stainless steel plate used for a brake disk of a two-wheeled vehicle and a producing method of the stainless steel plate, more specifically, to a martensitic stainless steel plate used for a brake disk of a two-wheeled vehicle, the martensitic stainless steel plate having excellent properties of a surface and an end surface.

BACKGROUND ART

The brake disk of a two-wheeled vehicle is required to have properties such as wear resistance, corrosion resistance, and toughness. In general, wear resistance is increased as hardness is increased. However, since an excessively high hardness causes a so-called brake squeak between a brake and a pad, the hardness of the brake is required to range from 32 to 38 HRC (Rockwell hardness C-scale). Because of these demanded properties, a martensitic stainless steel plate is used for the brake disk of the two-wheeled vehicle.

Typically, SUS420J2 has been quenched and tempered to be adjusted to have a desired hardness, thereby providing a brake disk. In this case, two heat treatment processes of quenching and tempering have been adversely required. In order to solve this problem, Patent Literature 1 discloses an invention relating to a steel composition capable of stably obtaining a desired hardness in a wider quenching temperature range than that of a typical steel of a SUS420J2 steel, the steel composition being usable in the as-quenched state. Similarly to SUS410, SUS403 and SUS410S steels, the invention of Patent Literature 1 is provided by reducing a C content and compensating narrowing of the austenitic single phase temperature range caused by the reduction of the C content, in other words, narrowing of the quenching temperature range, by adding Mn that is an austenite stabilizing element.

Moreover, Patent Literature 2 discloses an invention relating to a steel sheet for a motorbike disk brake, the steel sheet being a low Mn steel and is used in the as-quenched state. This steel sheet is obtained by reducing Mn and simultaneously adding Ni and Cu having the same effect as an austenite forming element.

Recently, also in the two-wheeled vehicle, a reduction in a weight of a vehicle body has been demanded and a reduction in a weight of the two-wheeled-vehicle brake disk has been studied. In this case, a problem that is disk deformation due to softening of a disk material caused by heat generation at the time of braking is caused. In order to solve the problem, it is necessary to improve heat resistance of the disk material. One of solutions of the problem is to improve temper softening resistance. Patent Literature 3 discloses an invention relating to a method for improving heat resistance by adding Nb and Mo. Patent Literature 4 discloses an invention relating to a disk material having an excellent heat resistance obtained by subjecting the disk material to a quenching treatment at a temperature higher than 1000 degrees C.

As a brake disk having an excellent temper softening resistance, Patent Literature 5 discloses a brake disk having a martensitic structure in which a prior austenite grain has an average grain size of 8 μm or more, and Patent Literature 6 discloses an invention in which martensite accounts for 75% or more and Nb accounts for from 0.10% to 0.60% at an area ratio of a quenched structure.

Patent Literature 7 discloses control of components to a limited range in which cracks are unlikely to be generated, since the above low-C martensitic stainless steel has a low hot workability and easily causes a so-called cracked edge at a widthwise end at the time of hot rolling.

Patent Literature 8 relates to a manufacturing method of a ferritic stainless steel strip. Particularly, Patent Literature 8 discloses optimum conditions for sheet bar heating in relation to a manufacturing method with a high productivity of a ferritic stainless hot-rolled steel strip having excellent moldability and material uniformity.

CITATION LIST Patent Literature(S)

Patent Literature 1: JP-A-57-198249

Patent Literature 2: JP-A-8-60309

Patent Literature 3: JP-A-2001-220654

Patent Literature 4: JP-A-2005-133204

Patent Literature 5: JP-A-2006-322071

Patent Literature 6: JP-A-2011-12343

Patent Literature 7: JP-A-2008-285692

Patent Literature 8: JP-A-2000-61524

SUMMARY OF THE INVENTION Problem(s) to be Solved by the Invention

Because of these techniques, a low-C martensitic stainless steel has been widely used for a disk brake of a two-wheeled vehicle. On the other hand, an improvement in productivity in manufacturing the disk brake has been demanded in recent years. For instance, a reduction in a heating time for heating and quenching and a reduction in a polishing time after heating and quenching have been demanded. Moreover, an improvement in a yield by even using a widthwise end of a steel strip also has been demanded.

When a polishing amount per a unit time is increased in order to reduce the polishing time, unfavorably, a jig is increasingly worn and temper softening of a material occurs by heat generated by friction caused during a processing. Accordingly, in order to reduce the polishing time without heat generation by friction caused during the processing, a polishing thickness is generally decreased. Here, an edge seam defect at the widthwise end of the steel strip has been problematic.

FIG. 1A shows an appearance of an edge seam defect in an actual product. FIG. 1B shows a microscope photograph of a cross section of the edge seam defect in the actual product. A typical manufacturing process of a hot-rolled steel strip includes: heating a slab having a thickness from 150 mm to 250 mm to a temperature ranging from 1100 degrees C. to 1300 degrees C.; rolling the slab to form a rough bar having a thickness from 20 mm to 40 mm using a rough hot rolling mill; subsequently, rolling the rough bar to form a plate having a thickness from 3 mm to 6 mm using a finish hot rolling mill; and coiling the obtained plate. Since tension is not applied during the rough hot rolling, the slab expands widthwise, so that a part of an end surface of the slab becomes a surface of the rough bar. Since the end surface of the slab does not contact with the rolling roller at the beginning of the rough hot rolling, roughness of the end surface of the slab is large, which causes defects when the end surface of the slab is brought into contact with the rolling roller.

The edge seam defect is often observed in a hot-rolled steel strip of a steel material. FIG. 2 is a photograph showing an end surface of a 20-mm-thick steel ingot obtained by hot-rolling an original 80-mm-thick steel ingot of each of various stainless steels in a laboratory. It is understood that the stainless steels are considerably different from each other in a level of roughness of the end surface thereof. Moreover, it is understood that roughness of the end surface of SUS410 steel is considerably changed depending on a hot-rolling heating temperature. Since roughness of the end surface of the slab in the rough hot rolling is caused by a difference in a deformation pattern caused by a difference in a crystal orientation between crystal grains of the slab, the roughness becomes noticeable when the crystals grains are large-sized. For instance, in the course of cooling to the room temperature after solidification, a common steel is transformed twice of δ/γ and γ/α to have a fine structure. Herein, δ refers to δ ferrite, γ refers to austenite, and a refers to α ferrite. It should be noted that the expression of “ferrite” usually means a ferrite. δ ferrite is ferrite precipitated at A4 transformation point or higher. α ferrite is ferrite precipitated at A3 transformation point or lower.

Since the structure of the common steel is micronized by another transformation of α/γ in the hot-rolling heating and the rough hot rolling is performed in a γ single phase in which recrystallization easily occurs, the structure of the common steel becomes finer also with the effect of micronizing crystal grains by recrystallization, so that edge seam defect is unlikely to occur. On the other hand, as in the ferritic stainless steel, when ferrite grains are kept in a state as at the solidification until the hot-rolling heating without a single transformation, the edge seam defect is likely to occur due to a large grain size. In general, δ ferrite is not differentiated from a ferrite in a steel not forming a γ single phase after solidification as in the ferritic stainless steel.

As long as components are 13% Cr-0.2% C as those in SUS420J1 even in a martensitic stainless steel, an austenitic single phase is formed in hot-rolling heating, so that an edge seam defect is unlikely to occur due to a microstructure obtained by transformation and a microstructure obtained by recrystallization of austenite.

However, since a temperature range in which the low-C martensitic stainless steel exhibits the austenitic single phase is narrow, the low-C martensitic stainless steel has a duplex structure of δ ferrite and austenite in hot-rolling heating. The edge seam defect is likely to occur because of the δ ferrite at this time. Accordingly, the polishing thickness exceeding a depth of the edge seam defect is required in the polishing process after quenching of the disk brake, thereby hampering productivity.

When the hot-rolling heating temperature is decreased and an austenite ratio is increased, deformation resistance is increased to deteriorate hot workability, resulting in generation of a cracked edge in hot rolling. When the C amount is increased to raise the γ phase fraction, a quenching hardness becomes excessively high. When the austenite stabilizing elements such as Mn, Ni and Cu are further added, the material cost is increased and an annealing cooling time is prolonged in an annealing process of a hot-rolled plate, thereby hampering productivity. When the Cr amount is decreased to raise the austenite fraction, corrosion resistance is hampered.

Although it is required to know a change in the δ ferrite fraction in the course of the hot rolling in order to control the δ ferrite fraction, it is impossible to measure the δ ferrite fraction of the hot-rolled plate. In case of the slab in the hot-rolling heating, the δ ferrite fraction is measurable according to a phase diagram calculation method or a heat treatment test in a laboratory. When the duplex structure of austenite and martensite is rapidly cooled, since the austenitic phase forms a martensitic structure, the δ ferrite phase is easily distinguished from the martensitic structure as a less deformed δ ferrite phase. However, in the actual hot rolling process, it has been impossible to know a change in the δ ferrite amount in the course of the hot rolling after the slab is taken out of a hot-rolling heating furnace. The hot-rolled steel strip coiled after the finish hot-rolling exhibits a low toughness because of including the martensitic structure obtained by transformation of austenite. Accordingly, it is difficult to uncoil the hot-rolled steel strip without any treatment. The hot-rolled steel strip can be uncoiled by being subjected to a hot-rolling and annealing in a box annealing furnace to temper martensite to ferrite and carbide. However, it is impossible to examine a structure of the hot-rolled plate before annealing. The structure of the plate after hot-rolling and annealing is a structure of ferrite and carbide as shown in FIG. 3. It is impossible to measure the δ ferrite fraction.

Means for Solving the Problem(s)

The inventors studied an examination method of the δ ferrite fraction in a ferrite mother phase of the hot-rolled and annealed steel plate of the low-C martensitic stainless steel. As a result of testing various etching liquids for structure analysis according to Electron Backscatter Diffraction (EBSD) and observation using an optical microscope, it was found that the δ ferrite can be colored using a Murakami reagent. The Murakami reagent is an aqueous solution of potassium ferricyanide. The solution is heated and a sample is immersed in the solution to be etched. The Murakami reagent is usually used for distinguishing austenite from the δ ferrite phase by coloring the δ ferrite mixed in an austenite mother phase as seen in a solidification structure of the austenitic stainless steel. Although it was not expected at first that it became possible to distinguish the δ ferrite in the hot-rolled and annealed steel plate of the martensitic stainless steel in which the δ ferrite and ferrite were mixed, the δ ferrite was able to be clearly distinguished as shown in FIG. 4. The δ ferrite is shown by a gray contrast part in FIG. 4. Although a mechanism of distinguishing the δ ferrite by coloring the δ ferrite using the Murakami reagent is not clarified, as a result of the examination by the inventors, it is inferred that a high-Cr δ ferrite phase is colored by the Murakami reagent to be distinguished since the δ ferrite phase and the austenite phase (the martensite phase at the room temperature) are different from each other in a Cr concentration by about 1.5% in the hot-rolling heating. No example with use of the Murakami reagent for appearance of the δ ferrite of such a low-Cr martensite stainless steel has been found. It is a new finding that a just about 1.5% difference in the Cr amount is distinguished. Use of this method reveals a behavior of the δ ferrite which has not been known. For instance, it was found that the ferrite amount of the hot-rolled plate is considerably decreased as compared with the δ ferrite amount at the hot-rolling heating temperature. Moreover, the ferrite amount at a widthwise end of the steel strip is larger than that at a widthwise center thereof. There seems to be a possibility that a temperature difference is generated between a widthwise end and a widthwise center of the slab and a possibility that the δ ferrite amount is increased by decarburization in a surface layer of the slab. With respect to the hot-rolled steel plate that is not subjected to hot-rolling-and-annealing, the δ ferrite can be distinguished by evaluating the steel plate sample as described above.

A relationship between the edge seam defect and the δ ferrite amount is shown in FIG. 5. No edge seam defect is observed in the austenitic stainless steel having 0% δ ferrite fraction. As the δ ferrite fraction is increased, a depth of the seam defect is increased. However, an increase amount of the depth of the seam defect is small until the δ ferrite fraction reaches 30%. However, it is found that, when the δ ferrite fraction exceeds 30%, the depth of the seam defect drastically becomes large.

On the other hand, a cracked edge at the end of the steel plate is likely to occur when the δ ferrite fraction falls below 5%. FIG. 6 shows forms of ends of (11% Cr, 12% Cr)-0.04% C-1.4% Mn-0.03% N steels respectively with the δ ferrite fractions of 4% and 20% after subjected to hot-rolling in a laboratory. When the δ ferrite fraction is low, an apparent cracked edge occurs.

As described above, in both of the hot-rolled and annealed steel plate and the hot-rolled steel plate, the δ ferrite fraction is strongly related to the depth of the edge seam defect and the cracked edge at the widthwise end of the steel plate. In case of the martensitic stainless steel in which the δ ferrite fraction is controlled, since the cracked edge is not observed and the depth of the edge seam defect is shallow, a grinding depth in a manufacturing process of the brake disk can be shallow, thereby improving productivity of the brake disk. Further, since the very end of the steel plate can be used, a yield is also improvable.

Thus, it is considered that, in a method of controlling the δ ferrite fraction considerably affecting a surface quality, controlling of (1) a chemical composition and (2) a hot-rolling heating temperature is effective. However, in the chemical composition (1), it is not preferable to control the δ ferrite fraction by C and N amounts since a quenching hardness necessary for the disk brake is not obtained. Moreover, since Si, Mn, Cr, Ni, Cu and the like affect a thickness of a hot-rolled scale, temper softening resistance and corrosion resistance and addition of a large amount of Si, Mn, Cr, Ni, Cu and the like increases alloy costs and the like, a range of the δ ferrite fraction controllable by the chemical composition (1) is limited. In controlling of the δ ferrite amount by the hot-rolling heating temperature (2), when the heating temperature is decreased to 1150 degrees C. or less in order to decrease the δ ferrite amount, a cracked edge is likely to occur because of a difference in strength between the austenite phase and the ferrite phase slightly left. Accordingly, an improvement in the surface quality by controlling the δ ferrite amount is not easy.

The inventors scrutinized the δ ferrite amount of the hot-rolled and annealed steel plate, hot rolling operational conditions and the chemical compositions, and found a method effective for satisfying the surface quality, prevention of the cracked edge, and hardness and corrosion resistance required as the disk brake. Specifically, it is necessary to heat the rough bar by induction heating or the like to increase a temperature of the rough bar, the rough bar provided between the rough hot-rolling and the finish hot-rolling, in order to control the components to satisfy (1) the δ ferrite amount in the hot-rolling heating and (2) various properties and in order to prevent (3) the δ ferrite amount from being decreased due to lowering of the temperature during the rough rolling after being taken out of the hot-rolling heating furnace.

Based on these findings, the hot-rolled steel plate and the hot-rolled and annealed steel plate of martensitic stainless steel used for a brake disk, in which the edge seam defect is reduced and the widthwise end of the hot-rolled steel strip is prevented from forming a cracked edge, and a controlling method of a structure of the steel plates can be provided.

The invention has been achieved based on the findings. A solution of the problem of the invention, specifically, a martensitic stainless steel (including a hot-rolled steel plate (which is not subjected to hot-rolling-and-annealing) and a hot-rolled and annealed steel plate)) used for a brake disk of a two-wheeled vehicle, and a manufacturing method of the martensitic stainless steel of the invention are described as follows.

(1) According to an aspect of the invention, a martensitic stainless steel used for a brake disk of a two-wheeled vehicle includes: in % by mass, C of 0.025% to 0.080%, Si of 0.05% to 0.8%, Mn of 0.5% to 1.5%, P of 0.035% or less, S of 0.015% or less, Cr of 11.0% to 13.5%, Ni of 0.01% to 0.50%, Cu of 0.01% to 0.08%, Mo of 0.01% to 0.30%, V of 0.01% to 0.10%, Al of 0.05% or less, and N of 0.015% to 0.060%; the balance being Fe and inevitable impurities; a DFE value defined by a formula (1) ranging from 5 to 30; and a δ ferrite fraction observed in a cross section structure ranging from 5% to 30% by an area ratio,

DFE=12(Cr+Si)−430C−460N−20Ni−7Mn−89  Formula (1).

In the formula (1), Cr, Si, C, N, Ni and Mn in the formula (1) respectively indicate contents. (2) With the above arrangement, the martensitic stainless steel further includes: in % by mass, one or two of Ti of 0.03% or less and B of 0.0050% or less. (3) With the above arrangement, the martensitic stainless steel further includes: in % by mass, Nb 0.30% or less. (4) With the above arrangement, the martensitic stainless steel further includes: in % by mass, one or two of Sn of 0.1% or less and Bi of 0.2% or less. (5) According to another aspect of the invention, a manufacturing method of the martensitic stainless steel includes: heating a rough bar at a temperature ranging from 10 degrees C. to 50 degrees C. between a rough hot rolling and a finish hot rolling. (6) With the above arrangement, the martensitic stainless steel is a hot-rolled steel plate that is not subjected to hot-rolling-and-annealing. (7) With the above arrangement, the martensitic stainless steel is a hot-rolled and annealed steel plate.

According to the control technology of the structure and the compositions of the invention, the hot-rolled steel plate and the hot-rolled and annealed steel plate used for the brake disk of a two-wheeled vehicle, in which the edge seam defect is reduced on the widthwise end of the hot-rolled steel strip and the widthwise end thereof is prevented from forming a cracked edge, can be obtained. The quality of each of the obtained steel plates is favorable in terms of an improvement in productivity and a yield of the brake disk.

BRIEF DESCRIPTION OF DRAWING(S)

FIG. 1A shows an appearance of an edge seam defect at a widthwise end of a hot-rolled and annealed steel strip of a martensitic steel used for a brake disk.

FIG. 1B shows a microscope image obtained by observing a cross section of the edge seam defect at the widthwise end of the hot-rolled and annealed steel strip of the martensitic steel used for the brake disk.

FIG. 2 is a photograph showing a surface of a widthwise end of a 20-mm-thick steel ingot obtained using a laboratory hot-rolling mill by rolling an original steel ingot cast in dimensions of 300 L×180w×80t (mm) in a laboratory, in order to demonstrate a generation process of the edge seam defect.

FIG. 3 is a photograph showing a structure (in which ferrite grains and carbides mainly appear) after being hot-rolled and annealed, the structure being a general cross-sectional structure of a hot-rolled and annealed plate of a 11% Cr-1% Mn-0.04% C-0.04% N steel. The structure has been subjected to etching by aqua regia for a short period of time.

FIG. 4 is a photograph showing a distribution of δ ferrite in a TD cross section of a hot-rolled and annealed steel strip of a 11% Cr-1% Mn-0.04% C-0.04% N martensitic stainless steel, in which an edge seam defect and a cracked edge are not observed, the photograph showing a δ ferritic structure of the hot-rolled and annealed plate having a favorable quality in terms of the edge seam defect and cracked edge.

FIG. 5 illustrates a relationship between the depth of the edge seam defect and the δ ferrite amount of a sample taken from each of several types of martensitic stainless steels used for the disk brake of a two-wheeled vehicle, in which each of the martensitic stainless steels was subjected to a change in the hot-rolling heating temperatures from 1100 degrees C. to 1280 degrees C. and hot-rolled to form a plate having a 3.8-mm thickness, the hot-rolled plate was annealed, subsequently, the hot-rolled coil was uncoiled and the sample was taken.

FIG. 6 is a photograph showing a relationship between a cracked edge on an end surface of a 3-mm-thick plate and a δ ferrite amount affecting the cracked edge, in which the plate is obtained by heating a 50-mm steel ingot of 11-to-12% Cr-0.04% C-0.5-to-1.4% Mn-0.03% N steel to 1250 degrees C. at a laboratory and subsequently hot-rolling the steel ingot into the 3-mm-thick plate.

DESCRIPTION OF EMBODIMENT(S)

Exemplary embodiment(s) of the invention will be described below. Firstly, a reason why a steel composition of a stainless steel plate in an exemplary embodiment is limited will be described. A mark % with respect to the composition means a mass % unless otherwise particularly indicated.

C: From 0.025% to 0.080%

C is an essential element for obtaining a predefined hardness of the stainless steel plate after quenching and is added in combination with N so as to attain a predefined hardness level. In the exemplary embodiment, an upper limit of C is defined to be 0.080% to maximize the effect of N without an excessive addition of C When C is added beyond the upper limit, the hardness becomes excessive to cause disadvantages such as brake squeak and reduction of toughness of the stainless steel plate. The upper limit of the C content is desirably 0.060% in terms of hardness control and an improvement in corrosion resistance. On the other hand, when the C content is less than 0.025%, N needs to be excessively added in order to obtain the hardness. Accordingly, a lower limit of the C content is defined to be 0.025%. The C content is desirably 0.040% or more in terms of stability of quenching hardness.

Si: From 0.05%% to 0.8%

Si is required for deoxidation at melting and refining process and is also useful for inhibiting generation of oxidized scale at a quenching heat process. Since an effect of Si is exerted at 0.05% or more, a Si content is set at 0.05% or more. However, since Si is mixed in a raw material such as molten pig iron and an excessive decrease of the Si content increases cost, the Si content is desirably 0.20% or more. Moreover, since Si narrows an austenite single phase temperature region to impair a quenching stability, the Si content is defined to be 0.8% or less. However, the Si content is desirably 0.6% or less in order to decrease an additive amount of the austenite stabilizing element and reduce the cost.

Mn: From 0.5% to 1.5%

Mn is an element to be added as a deoxidation agent and contributes to expansion of the austenite single phase region and an improvement in hardenability. Since an effect of Mn is clearly exerted at 0.5% or more, a Mn content is set at 0.5% or more. The Mn content is desirably 1.1% or more in order to stably obtain the hardenability. However, since Mn promotes generation of oxidized scale at quenching heating, an upper limit of the Mn content is defined to be 1.5% or less in order to increase abrasion load. Also in consideration of a decreased in corrosion resistance caused by grains (e.g., MnS), the Mn content is desirably 1.3% or less.

P: 0.035% or Less

P is an element contained as impurities in a raw material such as molten pig iron and a main raw material such as ferrochromium. Since P is a harmful element to toughness of a hot-rolled and annealed plate after quenching, a P content is defined to be 0.035% or less. P is preferably 0.030% or less. Since excessive reduction of P essentially requires use of a high purity raw material, leading to cost increase, a lower limit of P is preferably 0.010%.

S: 0.015% or Less

Since S forms sulfide inclusions and causes deterioration of a general corrosion resistance (whole surface corrosion and pitting corrosion) of a steel material and S decreases hot workability and increases cracked-edge sensitivity of the hot-rolled steel plate, the upper limit of the S content is preferably small and set at 0.015%. The upper limit is more preferably 0.008%. As the S content becomes smaller, corrosion resistance becomes more favorable. However, since the reduction of the S content causes an increase in desulfurization burden and an increase in production cost, the lower limit of the S content is preferably set at 0.001%.

Cr: From 11.0% to 13.5%

Cr is an essential element for ensuring oxidation resistance and corrosion resistance in the exemplary embodiment. The Cr content of less than 11.0% does not exert these effects, while the Cr content of more than 13.5% narrows the austenite single phase region to impair hardenability. Accordingly, the Cr content is set in a range from 11.0% to 13.5%. In consideration of stability of corrosion resistance, the Cr content is desirably 12.0% or more. Moreover, in consideration of press formability, the Cr content is desirably 13.0% or less.

Ni: From 0.01% to 0.50%

Ni is mixed as inevitable impurities in an alloy raw material of a ferritic stainless steel and is generally contained in a range from 0.01% to 0.10%. Moreover, Ni is an element effective for suppression of progress of pitting corrosion and the effect of Ni is stably exerted by the addition of 0.03% or more of Ni. Accordingly, a lower limit of a Ni content is preferably set at 0.03%. On the other hand, since a large added amount of Ni may deteriorate press formability due to solid-solution hardening in a hot-rolled and annealed steel plate, an upper limit of the Ni content is set at 0.50%. In consideration of alloy cost, the Ni content is desirably 0.15% or less.

Cu: From 0.01% to 0.08%

Cu is effective for improving corrosion resistance of the martensitic structure including δ ferrite and the effect of Cu is exerted at 0.01% or more. Moreover, a positive addition of Cu is occasionally performed in order to improve the hardenability as the austenite stabilizing element. However, since an excessive addition of Cu causes a decrease in the hot workability and an increase in the raw material cost, an upper limit of a Cu content is set at 0.08% or less. In consideration of generation of corrosion due to acid rain, a lower limit of the Cu content is desirably set at 0.02% or more. Moreover, in consideration of press formability of the hot-rolled and annealed steel plate, the Cu content is preferably 0.08% or less.

Mo: From 0.01% to 0.30%

Mo is effective for improving corrosion resistance of the martensitic structure including δ ferrite and the effect of Mo is exerted at 0.01% or more. Accordingly, a lower limit of a Mo content is set at 0.01%. Since Mo is effective for improving the hardenability and improving heat resistance after quenching, the Mo content is preferably 0.02% or more. The steel is occasionally tempered by heating after quenching to cause a decrease in the hardness. Herein, the improvement in heat resistance after quenching means a small decrease in the hardness, which is also referred to temper softening resistance. Although a disk brake for use is subjected to quenching, a disk material is heated by resistance heat generated at the time of braking in use. Accordingly, this property is important.

Since Mo is an element for stabilizing a ferrite phase and an excessive addition of Mo narrows the austenite single phase temperature region to impair the hardenability, an upper limit of the Mo content is set at 0.30% or less.

In order to improve heat resistance after quenching, a composite addition of Mo and Nb is desirable. When both of Mo and Nb are simultaneously added, the Mo content in a range from 0.05% to 0.20% and the Nb content in a range from 0.05% to 0.20% are particularly preferable.

V: From 0.01% to 0.10%

V is mixed as inevitable impurities in an alloy raw material of a ferritic stainless steel and is not easy to remove in the refining process. Accordingly, V is generally contained in a range from 0.01% to 0.10%. Moreover, V is an intentionally added element as needed since V forms a fine carbonitride to improve wear resistance of the brake disk and exerts an effect of improving corrosion resistance. Since an effect of V is stably exerted by the addition of 0.02% or more, a lower limit of a V content is preferably set at 0.02%, more preferably 0.03% or more. On the other hand, since an excessive addition of V may form large-sized precipitates, leading to deterioration of toughness after quenching, an upper limit of the V content is set at 0.10%. In consideration of the production cost and productivity, the V content is desirably set at 0.08% or less.

Al: 0.05% or Less

Al is an element to be added as a deoxidizing element and improve oxidation resistance. Since an effect of Al is exerted at 0.001% or more, a lower limit of an Al content is preferably set at 0.001% or more. On the other hand, since solid-solution hardening and formation of large-sized oxide inclusions may cause deterioration of toughness of the brake disk, an upper limit of the Al content is set at 0.05%. The Al content is preferably 0.03% or less. It is not a requisite to contain Al.

N: From 0.015% to 0.060%

N is one of very important elements in the exemplary embodiment. Similarly to C, N is an essential element for obtaining a predetermined hardness after quenching and is added in combination with N so as to attain a predetermined hardness level. In the case of quenching as the duplex structure of austenite and ferrite at the time of quenching heating, precipitation of Cr carbide, in other words, a sensitization phenomenon is likely to occur, leading to deterioration of corrosion resistance. However, addition of nitrogen enables suppression of precipitation of Cr carbide, which may exert the effect of improving corrosion resistance. Since the effect is exerted at 0.015% or more, a N content is set at 0.015% or more. On the other hand, since the effect becomes saturated at 0.060% and formation of defects such as air bubbles is likely to decrease a yield, an upper limit of the N content is set at 0.060%. In consideration of the improvement in corrosion resistance by reinforcing a passivation film, the N content is desirably 0.030% or more. Moreover, the N content is desirably 0.050% or less.

An amount of δ ferrite (δ ferrite fraction), which is represented by an area ratio, observed in a hot-rolled steel plate or a hot-rolled and annealed steel plate is defined in a range from 5% to 30%.

The amount of δ ferrite in the steel affects generation of the edge seam defect and hot-rolled cracked edge at the time of hot rolling. When the δ ferrite fraction is less than 5%, hot workability is reduced to easily cause the cracked edge. Accordingly, the δ ferrite fraction is set at 5% or more. On the other hand, when the δ ferrite fraction is more than 30%, a grain size is increased to easily generate the edge seam defect and a large polishing thickness of the brake disk is required to remove the edge seam defect by polishing in a polishing process after quenching of the brake disk. Accordingly, the δ ferrite fraction is set at 30% or less. It should be noted that the δ ferrite is observed in a cross section of the hot-rolled and annealed steel plate and the hot-rolled steel plate at the time of hot rolling, and evaluated through observation using a typical microscope. A structure etching of the δ ferrite is desirably performed by a method of immersing a sample in a solution provided by heating the Murakami reagent (an aqueous solution of potassium ferricyanide).

A DFE value defined by the formula (1) (DFE=12(Cr+Si)−430C−460N−20Ni−7Mn−89) is in a range from 5 to 20.

When the DFE value is low, the δ ferrite amount is decreased to increase a generating frequency of the cracked edge at hot rolling. Accordingly, the DFE value is set at 5 or more. When the DFE value is high, the δ ferrite amount is increased to easily cause the edge seam defect. Accordingly, the DFE value is set at 20 or less. It should be noted that Cr, Si, C, N, Ni and Mn in the formula (1) indicate the respective contents (% by mass).

Moreover, in the exemplary embodiment, in addition to the above elements, the following elements may be added in order to improve rust resistance, heat resistance, hot workability and the like.

Ti: 0.03% or Less

Ti, which forms carbonitride, is an element for suppressing a sensitization phenomenon by precipitation of chrome carbonitride and a decrease in corrosion resistance in a stainless steel. A Ti content is preferably 0.001% or more. However, since formation of a large-sized TiN causes deterioration of toughness and squeaking of the brake disk, an upper limit of the Ti content is set at 0.03% or less. In consideration of toughness in winter, the Ti content is desirably 0.01% or less. It is not a requisite to contain Ti.

B: 0.0050% or Less

B is an element effective for improving hot workability. Since the effect of B is exerted when a B content is 0.0002% or more, B may be added at 0.0002% or more. In order to improve hot workability in a wider temperature range, the B content is desirably set at 0.0010% or more. On the other hand, since excessive addition of B causes deterioration of hardenability due to composite precipitation of boride and carbide, an upper limit of the B content is set at 0.0050%. In consideration of corrosion resistance, the B content is desirably 0.0025% or less.

Nb: 0.3% or Less

Nb, which forms carbonitride, is an element for suppressing a sensitization phenomenon by precipitation of chrome carbonitride and a decrease in corrosion resistance in a stainless steel. A Nb content is preferably 0.001% or more. Further, Nb is an element for largely improving heat resistance after quenching. Herein, heat resistance means how the stainless steel is unlikely to be softened when receiving heat after quenching. In other words, heat resistance is also referred to as temper softening resistance.

However, since excessive addition of Nb forms NbN in the brake disk to adversely cause a decrease in toughness and squeaking of the brake disk, an upper limit of the Nb content is set at 0.3%.

In order to improve heat resistance after quenching, a composite addition of Mo and Nb is desirable. When both of Mo and Nb are simultaneously added, Mo in a range from 0.05% to 0.20% and Nb in a range from 0.05% to 0.20% are particularly preferable.

Sn: 0.1% or Less

Sn is an element effective for improving corrosion resistance after quenching. A Sn content is preferably 0.001% or more, more preferably 0.02% or more as needed. However, since excessive addition of Sn promotes edge cracking at hot rolling, the Sn content is preferably set at 0.10% or less.

Bi: 0.2% or Less

Bi is an element for improving corrosion resistance. Although the mechanism is not clarified, it is deduced that addition of Bi decreases probability that MnS becomes a starting point of corrosion generation because Bi micronizes MnS that is likely to be the starting point. The effect is exerted when Bi is added at 0.01% or more. Since the effect is only saturated when Bi is added at more than 0.2%, an upper limit of the Bi content is set at 0.2%.

In addition to the above-described elements, impurity elements are contained as long as the effect of the invention is not hampered. Not only the above-described P and S which are general impurity elements but also Zn, Pb, Se, Sb, H, Ga, Ta, Ca, Mg, Zr and the like are preferably reduced as much as possible. On the other hand, to the extent of the range to solve the problem of the invention, content rates of these elements are controlled. The respective contents are Zn≦100 ppm, Pb≦100 ppm, Se≦100 ppm, Sb≦500 ppm, H≦100 ppm, Ga≦500 ppm, Ta≦500 ppm, Ca≦120 ppm, Mg≦120 ppm, and Zr≦120 ppm.

In the hot rolling process, using an induction heater (bar heater) between a rough rolling and a finish rolling, a rough bar having a plate thickness from 20 mm to 40 mm is preferably heated at a temperature from 10 degrees C. to 50 degrees C. When the temperature for heating the rough bar is less than 10 degrees C., the δ ferrite amount is small to decrease hot workability, so that the cracked edge is likely to be generated. On the other hand, when the temperature for heating the rough bar is more than 50 degrees C., the δ ferrite amount is excessively large to increase a grain size, thereby increasing roughness of an end surface of the rough bar, so that deep edge seam defect is likely to be generated. The temperature of the rough bar is increased also by increasing a slab heating temperature prior to the hot rolling process instead of heating by a rough bar heater. However, since the grain size is increased when the heating temperature exceeds 1250 degrees C., the roughness of the end surface of the rough bar is increased during the rough rolling to deepen the edge seam defect. Accordingly, the heating temperature in the hot rolling is desirably 1250 degrees C. or less. When the heating temperature in the hot rolling is less than 1150 degrees C., deformation resistance of the austenite mother phase is increased and the δ ferrite amount is decreased, so that a δ ferrite phase in a small amount is concentrically deformed to decrease hot-rolling deformability, whereby a cracked edge is generated to decrease a yield. Accordingly, the heating temperature in the hot rolling is desirably 1150 degrees C. or more.

With the components and the δ ferrite fraction recited in the claims, a quality defined in the claims is attainable. The martensitic stainless steel for a brake disk for a two-wheeled vehicle can exert effects in both of a hot-rolled steel plate without being subjected to hot-rolling-annealing, and a hot-rolled and annealed steel plate.

Modification(s)

The effects of the invention will be described below with reference to Examples. However, the invention is by no means limited to conditions used in the following Examples.

In each of Examples, firstly, a steel having a component composition shown in Tables 1-1 and 1-2 was melted and cast to obtain a 200-mm-thick slab. This slab was heated to a temperature in a range from 1150 degrees C. to 1250 degrees C. and subsequently subjected to a rough hot-rolling and a finish hot-rolling to obtain a 4-mm-thick hot-rolled steel plate, which was coiled in a temperature range of 750 degrees C. to 900 degrees C. The coiled hot-rolled steel plate was heated in a range of an increasing temperature from 10 degrees C. to 50 degrees C. using a rough bar heater with use of induction heating between the rough hot-rolling and the finish hot-rolling. Subsequently, the coiled hot-rolled steel plate was annealed in a box annealing furnace. The box annealing furnace was heated up to a temperature range of 800 degrees C. to 900 degrees C. After scales of a surface of the hot-rolled and annealed steel plate were removed by shot blasting and pickling, the hot-rolled and annealed steel plate was evaluated in terms of the edge seam defect and the cracked edge. The edge seam defect was judged as “Pass” when a depth of the edge seam defect was less than 150 μm, in which a judgment S was given when the edge seam defect was not visually observed; and a judgment A was given when the edge seam defect was visually observed. When there was an edge seam defect having a depth of 150 μm or more, the edge seam defect was judged as “Fail” (a judgment C).

The cracked edge was judged as: “Pass” (a judgment A) when no cracked edge having a depth of 10 mm or more was generated; and “Fail” (a judgment B) when a cracked edge having a depth of 10 mm or more was generated. When the cracked edge was continuously generated, the cracked edge was judged as “Fail” (a judgment C).

A cross-sectional structure was observed using an optical microscope and a δ ferrite amount was measured by image analysis. δ ferrite appeared using the Murakami reagent.

Subsequently, the hot-rolled, annealed and pickled plate was quenched and a surface thereof was subjected to a polish finish (#80). A JIS surface hardness (quenching hardness) was evaluated by a Rockwell C-scale hardness tester. The plate having the surface hardness from 32 to 38 was judged as “Pass” and the plate not having the surface hardness from 32 to 38 was judged as “Fail.” A disk brake was quenched under conditions of heating at an average heating rate of about 50° C./s to reach 1000 degrees C., keeping the temperature for one second after reaching 1000 degrees C., and cooling at an average cooling rate of 70° C./s to reach the ambient temperature.

In order to evaluate heat resistance after quenching, after tempering at 500 degrees C. for one hour and the polish finish (#80) of the surface, a JIS surface hardness (quenching hardness) was evaluated by a Rockwell C-scale hardness tester. The disk having the surface hardness of less than 32 was judged as “Fail (B)” and the disk having the surface hardness of 32 or more was judged as “Pass (A).” Further, the disk was subjected to a test in which the tempering temperature was 530 degrees C. in the same manner as the above. The surface hardness of 32 or more was judged as “Pass (S)” and put in a column of “Temper softening resistance” of Tables 2-1, 2-2 and 2-3.

In order to evaluate corrosion resistance, after a polish finish (#600), the hot-rolled, annealed and pickled plate was subjected to a salt spray test for four hours (JIS Z 2371: “Salt Spray Test Method”) and a rust area ratio was measured and judged as “Fail (B)” at 10% or more and “Pass (A)” at less than 10%. Particularly, the rust area ratio at zero was judged as “Pass (S).”

With respect to polishing performance, the depth of the edge seam defect of 150 μm or less was judged as “Pass (A)” and the depth of the edge seam defect of more than 150 μm was judged as “Fail (B).”

For Comparative Examples, the same evaluation was performed on samples with compositions, heating conditions of quenching, and a δ ferrite area ratio of the hot-rolled and annealed plate, which were beyond the scope of the invention.

TABLE 1-1 Sample Content (mass %) Steel Others DFE No. C Si Mn P S Cr Ni Cu Mo V Al N (Ti, B, Nb, Sn, Bi) Value Examples A A1 0.025 0.28 1.05 0.025 0.004 11.5 0.04 0.01 0.02 0.03 0.003 0.040 15.1 A2 0.080 0.30 1.10 0.026 0.004 13.5 0.02 0.03 0.01 0.02 0.004 0.016 26.7 A3 0.045 0.05 1.07 0.024 0.005 12.3 0.05 0.02 0.02 0.02 0.002 0.035 15.3 A4 0.060 0.80 0.55 0.027 0.003 12.5 0.04 0.02 0.01 0.05 0.004 0.028 27.3 A5 0.040 0.30 0.50 0.026 0.006 12.2 0.05 0.01 0.02 0.04 0.003 0.033 0.1% Sn 24.1 A6 0.050 0.38 1.50 0.025 0.004 12.1 0.02 0.02 0.02 0.03 0.004 0.036 11.8 A7 0.065 0.57 0.60 0.035 0.002 12.2 0.06 0.01 0.01 0.02 0.005 0.040 0.03% Ti 12.5 A8 0.045 0.60 1.01 0.028 0.015 12.0 0.05 0.02 0.02 0.01 0.004 0.036 18.2 A9 0.042 0.35 1.40 0.024 0.003 11.0 0.05 0.03 0.02 0.01 0.001 0.015 11.4 A10 0.041 0.30 1.50 0.035 0.003 12.1 0.05 0.02 0.02 0.02 0.004 0.035 14.6 A11 0.043 0.41 1.30 0.025 0.015 12.2 0.50 0.02 0.01 0.03 0.003 0.035 0.10% Nb 8.6 A12 0.038 0.40 1.20 0.021 0.004 11.5 0.03 0.08 0.02 0.02 0.001 0.041 9.6 A13 0.035 0.35 1.50 0.025 0.003 12.1 0.05 0.05 0.30 0.02 0.003 0.060 6.2 A14 0.050 0.40 1.21 0.025 0.002 11.5 0.02 0.03 0.01 0.10 0.004 0.040 0.005% B 5.0 A15 0.035 0.40 0.80 0.026 0.005 11.3 0.03 0.03 0.02 0.02 0.05 0.052 0.02% Ti 6.2 A16 0.052 0.60 1.10 0.028 0.005 12.5 0.03 0.05 0.03 0.04 0.005 0.050 0.05% Sn, 0.02% Ti 14.5 A17 0.041 0.50 1.05 0.026 0.005 11.4 0.03 0.02 0.03 0.03 0.014 0.050 5.2 A18 0.040 0.58 1.20 0.024 0.005 11.5 0.05 0.03 0.02 0.04 0.003 0.040 0.01% Nb 11.0 A19 0.041 0.41 1.20 0.025 0.001 12.4 0.05 0.01 0.02 0.02 0.003 0.035 0.005% B 21.6 A20 0.041 0.41 1.20 0.025 0.001 12.4 0.05 0.01 0.02 0.02 0.003 0.035 0.25% Nb 21.6 A21 0.037 0.42 1.20 0.025 0.001 11.7 0.05 0.01 0.20 0.02 0.003 0.035 0.15% Nb 15.0 A22 0.034 0.38 1.20 0.025 0.001 11.8 0.05 0.01 0.07 0.02 0.003 0.035 0.1% Bi 17.0 A23 0.038 0.41 1.20 0.025 0.001 12.2 0.05 0.01 0.05 0.02 0.003 0.035 0.05% Sn, 0.05% Bi 20.5 A24 0.042 0.41 1.20 0.025 0.001 12.4 0.05 0.01 0.15 0.02 0.003 0.035 0.005% B, 0.15% Nb, 21.2 0.05% Sn A25 0.041 0.38 1.20 0.025 0.001 11.7 0.05 0.01 0.10 0.02 0.003 0.035 0.005% B, 0.15% Nb, 12.8 0.05% Bi A26 0.039 0.41 1.20 0.025 0.001 11.5 0.05 0.01 0.20 0.02 0.003 0.035 0.005% B, 0.15% Nb, 11.7 0.05% Sn, 0.05% Bi

TABLE 1-2 Sample Content (mass %) Steel Others DFE No. C Si Mn P S Cr Ni Cu Mo V Al N (Ti, B, Nb, Sn, Bi) Value Comparatives B B1 0.085 0.44 0.80 0.025 0.003 12.1 0.04 0.02 0.01 0.02 0.002 0.025  7.0 B2 0.042 0.85 1.05 0.025 0.003 12.0 0.06 0.01 0.02 0.02 0.002 0.050 15.6 B3 0.043 0.04 1.40 0.024 0.005 12.2 0.05 0.02 0.02 0.02 0.005 0.021 18.9 B4 0.039 0.33 0.40 0.026 0.010 12.2 0.02 0.01 0.01 0.02 0.002 0.020 32.2 B5 0.020 0.31 1.60 0.025 0.005 12.4 0.01 0.02 0.02 0.02 0.005 0.020 34.3 B6 0.040 0.30 1.05 0.036 0.004 12.5 0.02 0.02 0.03 0.01 0.002 0.040 21.3 B7 0.048 0.31 0.80 0.020 0.020 12.6 0.04 0.01 0.02 0.02 0.10  0.025 27.4 B8 0.042 0.28 1.20 0.025 0.003 10.5 0.06 0.02 0.02 0.02 0.003 0.030 −1.1 B9 0.040 0.32 1.18 0.021 0.003 13.6 0.05 0.03 0.02 0.02 0.006 0.047 30.0 B10 0.041 0.30 1.30 0.027 0.005 12.4 0.00 0.02 0.02 0.02 0.005 0.035 20.6 B11 0.043 0.32 1.28 0.026 0.003 12.3 0.60 0.01 0.02 0.01 0.005 0.037  6.0 B12 0.044 0.29 1.20 0.025 0.003 12.5 0.06 0.00 0.02 0.02 0.002 0.040 17.6 B13 0.045 0.30 1.20 0.025 0.006 12.3 0.05 0.10 0.02 0.02 0.004 0.025 22.0 B14 0.044 0.31 1.20 0.027 0.003 12.3 0.07 0.01 0.00 0.02 0.008 0.032 18.9 B15 0.040 0.32 1.20 0.024 0.002 12.2 0.04 0.02 0.40 0.02 0.005 0.044 14.6 B16 0.038 0.33 1.20 0.026 0.003 12.4 0.08 0.01 0.02 0.00 0.004 0.040 19.0 B17 0.040 0.35 1.08 0.025 0.004 12.6 0.01 0.02 0.02 0.20 0.003 0.036 24.9 B18 0.048 0.32 1.30 0.025 0.004 12.3 0.05 0.02 0.02 0.02 0.004 0.010 27.1 B19 0.046 0.30 1.30 0.026 0.003 13.5 0.05 0.01 0.01 0.02 0.003 0.065 16.8 B20 0.068 0.36 1.50 0.027 0.004 12.7 0.05 0.02 0.02 0.05 0.004 0.050  4.0

TABLE 2-1 Rough bar Sample heater δ ferrite Edge seam Cracked Quenching Temper Salt steel heating temp. area ratio defect edge Polishing hardness softening spray Other NO. No. (° C.) (%) evaluation evaluation performance (HRC) resistance test properties Examples 1 A1 10 13 A A A 32 A A 2 A2 10 30 A A A 37 A A 3 A3 10 15 A A A 34 A A 4 A4 10 30 S A A 36 A A 5 A5 10 24 A A A 33 A S 6 A6 10 12 A A A 35 A A 7 A7 10 15 A A A 37 A A 8 A8 10 18 A A A 34 A A 9 A9 50 11 A A A 32 A A 10 A10 10 15 A A A 33 A A 11 A11 10 9 A A A 34 S A 12 A12 10 10 A A A 33 A A 13 A13 10 6 A A A 34 A A 14 A14 10 5 A A A 35 A A 15 A15 10 6 A A A 34 A A 16 A16 10 15 A A A 36 A S 17 A17 10 5 A A A 34 A A 18 A18 10 11 A A A 34 A A 19 A19 10 22 A A A 33 A A 20 A20 10 22 A A A 35 S A

TABLE 2-2 Rough bar Sample heater δ ferrite Edge seam Cracked Quenching Temper Salt steel heating temp. area ratio defect edge Polishing hardness softening spray Other NO. No. (° C.) (%) evaluation evaluation performance (HRC) resistance test properties Examples 21 A21 10 15 A A A 34 S A 22 A22 10 17 A A A 33 A S 23 A23 10 20 A A A 34 A S 24 A24 10 21 A A A 33 S S 25 A25 10 13 A A A 34 S S 26 A26 10 12 A A A 34 S S 27 A17 50 30 S A A 34 A A 28 A17 30 25 A A A 34 A A 29 A17 20 20 A A A 35 A A 30 A6 50 30 S A A 35 A A 31 A6 40 25 A A A 35 A A 32 A17 10 7 A A A 34 A A 33 A10 40 25 A A A 33 A A 34 A1 30 20 A A A 32 A A 35 A12 50 28 S A A 33 A A 36 A13 50 26 A A A 34 A A 37 A11 50 30 S A A 34 A A

TABLE 2-3 Rough bar Sample heater δ ferrite Edge seam Cracked Quenching Temper Salt steel heating temp. area ratio defect edge Polishing hardness softening spray NO. No. (° C.) (%) evaluation evaluation performance (HRC) resistance test Other properties Comparatives 38 B1 10   7.0 A A A 39 A A 39 B2 0 16.0 A A A 31 B A 40 B3 0 19.0 A A B 33 A A 41 B4 20  32.0 B A A 32 A A 42 B5 0 34.0 B A B 29 B A 43 B6 0 21.0 A B A 34 A A poor toughness 44 B7 10  27.0 A C A 34 A A 45 B8 0  0.0 A C A 33 A B 46 B9 0 32.0 B A A 31 B A 47 B10 10  21.0 A A A 33 A B 48 B11 20   4.0 A A A 34 A S poor press formability 49 B12 0 18.0 A A A 34 A B 50 B13 20  22.0 A B A 33 A A poor press formability 51 B14 0 19.0 A A A 34 A B 52 B15 0 15.0 A A A 30 B S 53 B16 10  19.0 A A A 33 A B 54 B17 10  25.0 A A B 33 A A poor toughness 55 B18 10  27.0 A A A 31 B A 56 B19 0 17.0 A A A 39 B A 57 B20 0  4.0 A C A 38 A A 58 A17 0  3.0 A C A 34 A A 59 A17 60  40.0 C B A 34 A A 60 B9 40  35.0 C B A 30 B A 61 B9 5  4.0 A C A 31 B A 62 B9 60  35.0 C B A 30 B A

As is apparent from Tables 1-1, 1-2, 2-1, 2-2 and 2-3, in Examples of the invention, which had the component composition to which the invention was applied, and the δ ferrite area ratio was in a range from 5% to 30%, the quality of the edge seam defect was judged as “Pass” and the quality of the cracked edge was also judged as “Pass.” The quenching hardness, heat resistance, and corrosion resistance were also favorable. Further, when the heating temperature applied to the rough bar using the rough bar heater falls within the range of the invention, the depth of the edge seam defect was further reduced, so that a polishing time of the disk after quenching was reducible. Moreover, the quality of the cracked edge was further improved to be unrecognizable. On the other hand, in the component composition falling out of the scope of the invention, it was difficult to control the δ ferrite amount in the hot-rolled and annealed plate. One or more of the quality of the edge seam defect, the quality of the cracked edge, the quenching hardness, and the corrosion resistance after quenching were judged as “Fail.” From the above, it is understood that the properties of the brake disks in Comparatives are inferior.

Specifically, since the values of C and N were high in the test Nos. 38 and 56 and the values of C and N were low in the test Nos. 42 and 55, the quenching hardness was beyond the target range. Since the value of Si was low in the test No. 40 and the value of V was high in the test No. 54, the polishing performance in the polishing process after quenching was inferior. Since the δ ferrite amount of the hot-rolled and annealed plate each in the test Nos. 41, 42, 45, 46, 57, 58, 59, 60, 61 and 62 exceeds 30% or falls less than 5%, the quality of the edge seam defect or the cracked edge was evaluated as inferior. Since the value of P was high in the test No. 43, the value of S was high in the test No. 44, and the value of Cu was high in the test No. 50, the cracked edge was evaluated as inferior.

Since the values of Cr, Ni, Cu, Mo and V were low in the test Nos. 45, 47, 49, 51 and 53, corrosion resistance was inferior. Since the large amounts of Ni and Cu were added in the test Nos. 48 and 50, press formability was inferior. Since the value of Cr was high in the test Nos. 46 and 60 to 62 and the values of Si and Mo were high in the test Nos. 39 and 52, hardenability was decreased to decrease the quenching hardness. Moreover, since the material cost was high in the test Nos. 48, 50, 52 and 54, the produced samples were judged economically inferior. Moreover, since the DFE value was low in the test No. 57, the δ ferrite fraction was low and the cracked edge was inferior.

From the above results, the above findings can be confirmed and the grounds for limiting the above steel compositions and structures can be supported.

As is apparent from the above description, the martensitic stainless steel plate of the invention used for the brake disk is a high-quality brake disk having favorable qualities of the edge seam defect and the cracked edge and being free from deterioration in hardness and corrosion resistance after quenching, which attains optimization of the δ ferrite amount observed in the hot-rolled and annealed steel plate and the hot-rolled steel plate by controlling the component design and the hot rolling conditions. Moreover, the qualities of the edge seam defect and the cracked edge were further improved by heating the rough bar between the rough hot rolling and the finish hot rolling under the optimum conditions depending on the composition of each of the steel plates. A material to which the invention is applied is used for the two-wheeled brake disk, whereby a yield is improvable and an examination burden is reducible, and further, productivity is improvable due to a shortening of the polishing time, so that the invention can increasingly contribute to the society. In other words, the invention has a sufficient industrial applicability. 

1. A martensitic stainless steel used for a brake disk of a two-wheeled vehicle, the martensitic stainless steel comprising: in % by mass, C of 0.025% to 0.080%, Si of 0.05% to 0.8%, Mn of 0.5% to 1.5%, P of 0.035% or less, S of 0.015% or less, Cr of 11.0% to 13.5%, Ni of 0.01% to 0.50%, Cu of 0.01% to 0.08%, Mo of 0.01% to 0.30%, V of 0.01% to 0.10%, Al of 0.05% or less, and N of 0.015% to 0.060%; the balance being Fe and inevitable impurities; a DFE value defined by a formula (1) ranging from 5 to 30; and a δ ferrite fraction observed in a cross section structure ranging from 5% to 30% by an area ratio, DFE=12(Cr+Si)−430C−460N−20Ni−7Mn−89  Formula (1) where: Cr, Si, C, N, Ni and Mn in the formula (1) respectively indicate contents.
 2. The martensitic stainless steel according to claim 1, further comprising: in % by mass, one or two of Ti of 0.03% or less and B of 0.0050% or less.
 3. The martensitic stainless steel according to claim 1, further comprising: in % by mass, Nb 0.30% or less.
 4. The martensitic stainless steel according to claim 1, further comprising: in % by mass, one or two of Sn of 0.1% or less and Bi of 0.2% or less.
 5. A manufacturing method of the martensitic stainless steel according to claim 1, comprising: heating a rough bar at a temperature ranging from 10 degrees C. to 50 degrees C. between a rough hot rolling and a finish hot rolling.
 6. The martensitic stainless steel according to claim 1, wherein the martensitic stainless steel is a hot-rolled steel plate that is not subjected to hot-rolling-and-annealing.
 7. The martensitic stainless steel according to claim 1, wherein the martensitic stainless steel is a hot-rolled and annealed steel plate.
 8. The martensitic stainless steel according to claim 2, further comprising: in % by mass, Nb 0.30% or less.
 9. The martensitic stainless steel according to claim 2, further comprising: in % by mass, one or two of Sn of 0.1% or less and Bi of 0.2% or less.
 10. The martensitic stainless steel according to claim 3, further comprising: in % by mass, one or two of Sn of 0.1% or less and Bi of 0.2% or less.
 11. The martensitic stainless steel according to claim 8, further comprising: in % by mass, one or two of Sn of 0.1% or less and Bi of 0.2% or less.
 12. A manufacturing method of the martensitic stainless steel according to claim 2, comprising: heating a rough bar at a temperature ranging from 10 degrees C. to 50 degrees C. between a rough hot rolling and a finish hot rolling.
 13. A manufacturing method of the martensitic stainless steel according to claim 3, comprising: heating a rough bar at a temperature ranging from 10 degrees C. to 50 degrees C. between a rough hot rolling and a finish hot rolling.
 14. A manufacturing method of the martensitic stainless steel according to claim 4, comprising: heating a rough bar at a temperature ranging from 10 degrees C. to 50 degrees C. between a rough hot rolling and a finish hot rolling.
 15. A manufacturing method of the martensitic stainless steel according to claim 8, comprising: heating a rough bar at a temperature ranging from 10 degrees C. to 50 degrees C. between a rough hot rolling and a finish hot rolling.
 16. A manufacturing method of the martensitic stainless steel according to claim 9, comprising: heating a rough bar at a temperature ranging from 10 degrees C. to 50 degrees C. between a rough hot rolling and a finish hot rolling.
 17. A manufacturing method of the martensitic stainless steel according to claim 10, comprising: heating a rough bar at a temperature ranging from 10 degrees C. to 50 degrees C. between a rough hot rolling and a finish hot rolling.
 18. A manufacturing method of the martensitic stainless steel according to claim 11, comprising: heating a rough bar at a temperature ranging from 10 degrees C. to 50 degrees C. between a rough hot rolling and a finish hot rolling.
 19. The martensitic stainless steel according to claim 2, wherein the martensitic stainless steel is a hot-rolled steel plate that is not subjected to hot-rolling-and-annealing.
 20. The martensitic stainless steel according to claim 3, wherein the martensitic stainless steel is a hot-rolled steel plate that is not subjected to hot-rolling-and-annealing.
 21. The martensitic stainless steel according to claim 4, wherein the martensitic stainless steel is a hot-rolled steel plate that is not subjected to hot-rolling-and-annealing.
 22. The martensitic stainless steel according to claim 8, wherein the martensitic stainless steel is a hot-rolled steel plate that is not subjected to hot-rolling-and-annealing.
 23. The martensitic stainless steel according to claim 9, wherein the martensitic stainless steel is a hot-rolled steel plate that is not subjected to hot-rolling-and-annealing.
 24. The martensitic stainless steel according to claim 10, wherein the martensitic stainless steel is a hot-rolled steel plate that is not subjected to hot-rolling-and-annealing.
 25. The martensitic stainless steel according to claim 11, wherein the martensitic stainless steel is a hot-rolled steel plate that is not subjected to hot-rolling-and-annealing.
 26. The martensitic stainless steel according to claim 2, wherein the martensitic stainless steel is a hot-rolled and annealed steel plate.
 27. The martensitic stainless steel according to claim 3, wherein the martensitic stainless steel is a hot-rolled and annealed steel plate.
 28. The martensitic stainless steel according to claim 4, wherein the martensitic stainless steel is a hot-rolled and annealed steel plate.
 29. The martensitic stainless steel according to claim 8, wherein the martensitic stainless steel is a hot-rolled and annealed steel plate.
 30. The martensitic stainless steel according to claim 9, wherein the martensitic stainless steel is a hot-rolled and annealed steel plate.
 31. The martensitic stainless steel according to claim 10, wherein the martensitic stainless steel is a hot-rolled and annealed steel plate.
 32. The martensitic stainless steel according to claim 11, wherein the martensitic stainless steel is a hot-rolled and annealed steel plate. 